Patent GB-1524230-A (GB)
Title: Ghost signal eliminating system
IPC: H04N5/21
Filed: 1975-12-01
Abstract: 1524230 Ghost signal elimination, analogue-todigital conversion MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 1 Dec 1975 49293/75 Headings H4L and H3H In a ghost signal eliminating system, a variable delay circuit 5 (Fig. 1) is provided to delay an original video signal received at 1 by the delay time between a ghost and true image, the delayed signal being subtracted from the original signal (circuit 9). The variable delay circuit has coarse and fine adjustment circuits. The coarse adjustment is controlled from control unit 6 by a fixed voltage which appears at one of a number of output terminals depending on the level of a variable control voltage. The fine adjustment is controlled either by an output voltage from a comparator in which the fixed voltage is subtracted from the varying control voltage, or by a further fixed voltage developed at one of a plurality of output terminals of a level detection circuit, in accordance with the output of the comparator. In the embodiment of Fig. 1, the signal applied to the variable delay circuit 5 is that derived from the video IF stage of a TV receiver. After a delay set by the time control unit 6, the IF signal passes to a gain control circuit 7 and detector 8. The undelayed video IF is detected at 4, and the signals from detectors 4 and 8 are passed to circuit 9 for subtraction. Appropriate adjustment of delay in unit 6 and gain control 7 will eliminate any ghost. The variable delay device may be a supersonic delay line using surface waves or a charge transfer device such as a bucket brigade device or charge coupled device, and has a series of terminals via which input and output signals can be selectively routed to control the delay in both coarse and fine manner. Fine delay adjustment is achieved in a supersonic delay line by applying the input signal to one of a set of closely spaced input terminals via a control circuit which selects an appropriate terminal, by applying a pulse signal to one of a number of input lines (Fig. 3, not shown). Coarse adjustment is achieved by taking the delay line output signals from one of a number of output terminals, selected in a similar way (Fig. 4, not shown). The provision of pulses to select appropriate coarse and fine terminals is achieved by the circuit shown in Fig. 6, which incorporates a level detection part 16 producing an output at D, E or F depending on the level of the control voltage at 15, a voltage comparator 17 which converts the output D, E or F to an incremental level voltage and subtracts it from the control voltage and a level detector 18 producing an output at A, B or C depending on the level of the output from the comparator 17. In an alternative embodiment, a received video signal is first detected before being applied to the variable delay circuit and subtractor circuit (Fig. 7, not shown). Where a bucket brigade device delay system is used, fine control is by means of a voltage controlled oscillator which delivers controllable timing pulses which control charge transfer through the bucket brigade. Coarse control is achieved by switching the number of stages of the BBD (Fig. 8, not shown). Fine control is achieved by applying a continuously variable voltage to the VCO, derived from subtracting the coarse control output from a continuously variable control input (Fig. 9, not shown).